Fig. 1

The evolutionary radiation of perissodactyls, showing the major branches of the horses, rhinos, tapirs, chalicotheres, bronthotheres, and other extinct groups. As can be seen from the crown views of the upper left cheek teeth, the details of the crests and cusps are extremely similar between Radinskya, the early brontothere Palaeosyops, the primitive horse Protorohippus (long called “Hyracotherium”), the primitive moropomorph Homogalax, the chalicothere Litolophus, the tapiroid Heptodon, and the primitive rhinoceros Hyracodon. Shown next to the upper cheek teeth are typical skulls of horses, tapirs, and rhinos, emphasizing how similar they all looked in the early stages of perissodactyl evolution. The numbered branching points are as follows: 1 Perissodactyla, 2 Titanotheriomorpha, 3 Hippomorpha, 4 Moropomorpha, 5 Isectolophidae, 6 Chalicotheroiodiea, 7 Tapiroidea, 8 Rhinocerotoidea (phylogeny after Prothero and Schoch 1989; diagram after Kemp (2005), Fig. 7.19, p. 261; used by permission)